The present invention relates in general to semiconductor devices and, more particularly, to low frequency filter networks formed on semiconductor substrates.
Wireless communications devices typically operate using both radio frequency (RF) signals and lower frequency audio signals. For example, cellular telephones transmit RF carrier signals that operate at frequencies of six gigahertz or more and are modulated with audio frequency voice information. A microphone generates an audio frequency signal from the voice information which is amplified and used to modulate the RF carrier signal. Most wireless communications devices use a low pass filter at the microphone input to suppress ambient RF carrier signals that may be “picked up” or detected by the microphone in order to avoid degrading the performance of the communications device by noisy operation, loop instability, or other effects that reduce the quality of the modulating audio signal. To accomplish this function, the low pass filters have a passband in the audio range, i.e., less than about twenty kilohertz.
Presently, these audio filters are formed with discrete passive components because of the difficulty of forming the large component values that set the filters' low frequency passband. However, the discrete filters add a substantial fabrication cost to a wireless device. Integrated filters based on semiconductor technology have a lower cost but have not been practical because of the large die area needed to integrate audio frequency components while providing an adequate voltage capability.
Hence, there is a need for an integrated filter that provides a high level of frequency selectivity while maintaining a low manufacturing cost.